1. Field of the Invention
The present invention relates to a controller for an AC power converter comprising a filter connected to a load through an output bus.
2. Description of the Prior Art
As an example of a controller for an AC power converter of the prior art, Transaction of Semiconductor Power Conversion Group SPC-86-59, Japan Electrical Engineering Society reports "Waveform Control System of Constant Frequency Sine Wave Inverter". FIG. 1 is a block diagram of the structure of this control system.
In FIG. 1, the reference numeral 100 denotes an inverter body; 102, a reactor; 103, a capacitor forming an LC filter with the reactor 102; 121, a current controller; 122, a subtractor; 123, a limiter for limiting current commands; 124, an adder; 126, a voltage controller; 127, a reference capacitor current generator; 128, a subtractor; 129, an output voltage reference generator; 131, a load current detector; 134, a PWM modulation circuit; 151, a clock circuit.
Next, operations will be explained. The inverter body 100 is controlled by the PWM circuit 134 according to an output of the current controller 121 as voltage command and supplies AC power to a load through an LC filter of reactor 102 and capacitor 103. This figure indicates a single phase inverter; however the principle is the same for a 3-phase inverter.
An output current i.sub.f of inverter body 100 is controlled at a high speed by taking the deviation of i.sub.f from a detected current signal in the subtractor 122 using an output current i* of the current command limiter 123 and generating a voltage command through the current controller 121 in accordance with the deviation. Therefore, an over current condition by a sudden change of load impedance can be suppressed to protect power conversion elements of the inverter body 100. This output current command is expressed by a sum of load current and capacitor current of the LC filter to be supplied for establishing output bus voltage. Accordingly, a reference capacitor current is calculated by the signal of clock circuit 151 synchronized with the output bus voltage through the reference capacitor current generator 127 and the output current i.sub.0 * of this reference capacitor current generator 127 is added to the detected load current signal detected by the load current detector 131 in the adder 124 to generate a current command. In this case, deviation between the detected output voltage signal applied to the load and a reference voltage generated by the output voltage reference generator 129 is obtained by the subtractor 128 and this deviation is inputted to the voltage controller 126 for the purpose of voltage control. The voltage control characteristic can be improved by using a current command which is obtained by adding, as explained above, both the load current detecting signal from the load current detector 131 and the output current i.sub.0 * of the reference capacitor current generator 127 to the output of the voltage controller 126.
The control system of FIG. 1 can be expressed by the formulae as indicated below. EQU Viu=GI(s) (Iiu*-Iiu) (1) EQU Iiu*=Iou+.omega.cEcos (.omega.t)+Gv(s) [Esin (.omega.t) -Vou](2)
Where, Viu is an output voltage of phase U of the inverter body 100, Vou is an output bus voltage of phase U, Iiu is an output current of phase U of the inverter body 100, Iiu* is an output current command of phase U of the inverter body 100, Iou is a load current of phase U, .omega. is an angular frequency of output bus voltage, E is an effective value command of output bus voltage, c is an electrostatic capacitance of capacitor, and GI(s), Gv(s) are control elements of current controller 121 and voltage controller 126 respectively.
The control system of FIG. 1 directly controls the AC detection signal. Since this system performs the so-called follow-up control, the integral element cannot be introduced into the control elements GI(s), Gv(s) and thereby a voltage waveform may be distorted depending on a load, resulting in malfunction of a load apparatus in some cases.
Moreover, as indicated below in addition to the prior art example shown in FIG. 1, control can be realized by converting an AC signal into A DC signal through a coordinate converter.
FIG. 2 shows, for example, a conventional controller for an AC power converter as disclosed in the specification of prior art application No. 63-133073 by the same applicant of the present invention. A No. 1 inverter 1 supplies a power signal to a load 4 in parallel with a No. 2 inverter 2 of the same structure through a bus 3.
The elements of this No. 1 inverter similar to those of FIG. 1 are given like reference numerals and their explanation is not repeated here. In FIG. 2, 104 is an output switch; 120, an adder; 125, a limiter; 132, a voltage detector; 133, a current detector; 135, a shared current detector; 140, 2-phase/3-phase converter; 141, 142, 143, 3-phase/2-phase converter; 149, a PLL (phase locked loop) circuit; 150, 3-phase sine wave generator; 160, 161, CT (current transformer).
Operations of the controller for the AC power converter shown in FIG. 2 will now be explained. The No. 1 inverter 1 is mainly composed of an inverter body 100, a reactor 102 and a capacitor 103 to convert DC power of from DC power supply 5 into AC power and to output the AC power to the output bus 3 and supply the a power to the load 4.
The inverter current is detected by CT 160 and current detector 133, while the load current is detected by CT 161a and the output bus voltage is detected by a voltage detector 132. These voltages and currents are detected as AC values but when these are mapped onto the orthogonal coordinate system which rotates in synchronism with the basic wave, the basic wave can be treated as a DC value and thereby control can be made easy. Therefore, the reference sine wave for coordinate conversion is generated from the output bus voltage by using PLL 149 and sine wave generator 150 and those currents and voltages are converted to DC values by controlling the 3-phase/2-phase converters 141, 142, 143 with such reference sine wave.
The reference output voltage generator 129 outputs a line effective value command of the output bus voltage. Deviation between this line effective value command and the output bus voltage of 3-phase/2-phase converter 142 is calculated using the subtractor 128 and a current command for correcting voltage deviation is generated by the voltage controller 126. Moreover, since a load current must be applied to a capacitor to cause it to generate a constant AC voltage, the reference current is generated by the reference capacitor current generator 127.
In the example of FIG. 2, since the inverters are connected in parallel to the output bus 3, the shared current detector 135 determines a load current command so that each inverter shares the load current. This load current command is restricted by the limiter 125 to prevent the load current command from exceeding the capacity of the inverter. The load current command from the limiter 125 the output of reference capacitor current generator 127, and the output of voltage controller 126 are added up in the adder 124 to obtain an inverter current command value through the limiter 123.
The inverter current control system takes the deviation between an inverter current from the 3-phase/2-phase converter 141 and a command value of inverter current from the limiter 123 in the subtractor 122 and adds an output of the current controller 121 to the output bus voltage from the 3-phase/2-phase converter 142 in the adder 120 to obtain the inverter voltage command. This inverter voltage command is converted back to a 3-phase AC voltage command in the 2-phase/3-phase converter 140 and corresponding voltage is outputted from the inverter body 100 through PWM control of the PWM circuit 134.
The control system of FIG. 1 can be further expressed by the formulae (3) and (4) below.
The formulae (3), (4) can be applied to the inverter including an LC filter. Voltages and currents indicate the values on the rotatable coordinate axes; V.gamma.i, V.delta.i are inverter voltages; V.gamma.o, V.delta.o are output bus voltages; l.gamma.i, I.delta.i are inverter currents; I.gamma.o, I.delta.o are load currents. C, G are capacitance of capacitor and conductance respectively; L, R are reactance of reactor and resistance respectively. ##EQU1##
Meanwhile, the control system of FIG. 2 can be expressed by the formulae (5) and (6). ##EQU2## Where, V.gamma.o*, V.delta.o* are .gamma. axis and .delta. axis output bus voltage commands respectively.
It is understood that V.gamma.o, V.delta.o are added to the output of current controller in the formula (5) because V.gamma.o, V.delta.o become a disturbance on the current control as seen in the formula (4) and therefore the influence thereof must be compensated. Moreover, the terms I.gamma.o, I.delta.o, .omega.CV.gamma.o and .omega.CV.delta.o in the formula (6) compensate for influence of both the disturbance caused by I.gamma.o, I.delta.o and the interference caused by .omega.CV.gamma.o, .omega.CV.delta.o in voltage control as seen in the formula (3). As explained above, improved control performance of current, voltage control system enables an inverter to prevent the over current condition due to change of load and fault of inverter.
A non-linear load such as a capacitor input type rectifier is often used as the load 4 and a current including harmonics often flows into the output bus.
As will be understood from the formula (3), fluctuation of load currents I.gamma.o, I.delta.o appear in directly as fluctuation of output bus voltage.
The formulae (3) and (4) may be transformed as follows. ##EQU3## Where ##EQU4##
Moreover, the formulae (5), (6) may be transformed as follows. ##EQU5##
Comparison of formulae (7), (8) with (10), (11) suggests that the control system of FIG. 2 compensates for output bus voltage components and capacitor current component in order to suppress fluctuation of the output bus voltage, but the output bus voltage happens to be distorted and results in influence such as malfunction of the load as in the case of the first example shown in FIG. 1 due to the following reasons.
1) Control of charging currents D.gamma., D.delta. based on formula (8) causes fluctuation of output bus voltage because output currents I.gamma.i, I.delta.i and charging currents D.gamma., D.delta. are included as the interference terms and the charging currents D.gamma., D.delta. fluctuate if such interference terms change during transition periods.
2) Since the formula (10) uses a voltage command for calculation of the following capacitor current command term, ##EQU6## if the voltage is deviated from the command value due to the influence of disturbance, a capacitor current is no longer controlled accurately, and the charging currents D.gamma., D.delta. change, resulting in fluctuation of output bus voltage.
In addition to above prior art, U.S. Pat. No. 4823251 applied by the same applicant of the present invention proposes an apparatus which realizes feed forward control by calculating the interference voltage component from current to voltage in a discrete-time system.